Learn by writing code

Beginner Beginner sets

• SQL Filtering and Sorting PracticePractise DISTINCT, ORDER BY, LIMIT, AND/OR/NOT, NULL checks and column aliases on a product catalogue.8 problems
• SQL Functions PracticePractise COUNT, SUM, AVG, MIN and MAX, string functions like UPPER and LENGTH, and filtering by date ranges.8 problems
• SQL GROUP BY PracticeSummarise data per group with COUNT, SUM and AVG, and filter groups with HAVING.3 problems
• SQL INSERT, UPDATE and DELETE PracticePractise changing data safely with INSERT, UPDATE and DELETE, and changing tables with CREATE TABLE and ALTER TABLE.7 problems
• SQL SELECT PracticePractise reading data with SELECT, choosing columns, calculating values, and filtering rows with WHERE.4 problems

Intermediate Intermediate sets

• SQL GROUP BY and HAVING PracticeGroup sales data by one or more columns, combine WHERE with HAVING, and filter groups by their totals, averages and counts.7 problems
• SQL JOIN PracticeCombine tables with INNER JOIN and LEFT JOIN, find missing matches, and summarise joined data.3 problems
• SQL JOIN Types PracticeGo beyond INNER and LEFT JOIN with RIGHT JOIN, FULL OUTER JOIN, self joins, cross joins, and queries that join three tables.7 problems
• SQL Subqueries, CASE and UNION PracticePractise subqueries, IN and EXISTS, ANY and ALL, CASE expressions, UNION and UNION ALL, and common table expressions (CTEs).10 problems

Advanced Advanced sets

• SQL Recursive CTEs, Views and Transactions PracticeGenerate series and walk hierarchies with recursive CTEs, save queries as views, and make changes safely with transactions and indexes.7 problems
• SQL Window Functions PracticeRank rows with ROW_NUMBER, RANK and DENSE_RANK, compare with group averages using PARTITION BY, and calculate changes and running totals with LAG, LEAD and SUM OVER.8 problems

Beginner Beginner sets

• Python Conditions PracticePractise if, elif and else, combined and nested conditions, and controlling loops with break, continue and pass.8 problems
• Python Dictionaries, Sets and Tuples PracticePractise counting and grouping with dictionaries, comparing groups with sets, and packing and unpacking tuples.9 problems
• Python Functions PracticePractise writing functions with parameters, return values and default arguments.3 problems
• Python Lists PracticePractise indexing, slicing, adding, removing and summarising list items.3 problems
• Python Loops PracticePractise for loops, while loops, range(), break and continue.3 problems
• Python Operators and Input PracticePractise arithmetic operators, operator precedence, type conversion and reading user input with input().8 problems
• Python Strings PracticePractise indexing and slicing strings, string methods like split, join, strip and replace, and formatting output with f-strings.8 problems
• Python Variables PracticePractise creating, updating and printing variables, and converting between types.3 problems

Intermediate Intermediate sets

• Practical Python PracticePractise real-world tasks, working with SQLite databases, CSV files, folders of files, JSON from APIs, and HTML pages, all without needing an internet connection.7 problems
• Python Files, Exceptions and Modules PracticePractise reading and writing files, handling errors with try/except, and using the json, datetime, re and collections modules.9 problems
• Python Functions Deep Dive PracticePractise default and keyword arguments, *args and **kwargs, returning several values, scope, lambda functions and recursion.10 problems
• Python OOP PracticePractise classes and objects, constructors, inheritance, polymorphism, encapsulation with properties, and abstract base classes.8 problems

Advanced Advanced sets

• Advanced Python PracticePractise comprehensions, iterators, generators, decorators and context managers, the features that make Python code short and expressive.9 problems

Beginner Beginner sets

• C Basics PracticePractise C variables, operators, input and output, conditions, loops and functions with short programs.10 problems

Intermediate Intermediate sets

• C Arrays, Strings and Structures PracticePractise arrays, 2D arrays, C strings and the string.h functions, structs, files and bit manipulation in C.10 problems
• C Pointers and Memory PracticePractise pointers, pointer arithmetic, output parameters, malloc, realloc and free, and linked lists in C.8 problems

Advanced Advanced sets

Advanced C sets are being written.

Beginner Beginner sets

• C++ Arrays, Vectors and Strings PracticePractise std::vector, std::array, std::string, 2D grids and structs with common collection tasks.7 problems
• C++ Basics PracticePractise C++ variables, data types, arithmetic operators, integer division, type conversion and formatted output.7 problems
• C++ Control Flow PracticePractise if/else, switch, for and while loops, nested loops, break and continue in C++.7 problems
• C++ Functions PracticePractise writing C++ functions, passing by value and by reference, overloading, default arguments, recursion and static variables.7 problems

Intermediate Intermediate sets

• C++ Object-Oriented Programming PracticePractise C++ classes, constructors, encapsulation, inheritance, virtual functions, abstract classes and operator overloading.6 problems
• C++ References, Pointers and Memory PracticePractise references, pointers, nullptr checks, pointer arithmetic and smart pointers in C++.6 problems
• C++ STL PracticePractise std::map, std::set, std::unordered_map, stacks, priority queues and the standard algorithms on real tasks.7 problems

Advanced Advanced sets

• Advanced C++ PracticePractise lambdas, templates, exceptions, file streams, std::optional and move semantics in modern C++.7 problems

Beginner Beginner sets

• Java Arrays and Strings PracticePractise Java arrays, 2D arrays, String methods, StringBuilder and wrapper classes on everyday tasks.8 problems
• Java Basics PracticePractise Java variables, data types, arithmetic, integer division, casting, string conversion and formatted output.7 problems
• Java Control Flow PracticePractise if/else, switch expressions, for and while loops, nested loops, break and continue in Java.7 problems
• Java Methods PracticePractise writing static methods, return values, pass-by-value behaviour, overloading, varargs and recursion in Java.7 problems

Intermediate Intermediate sets

• Java Collections PracticePractise ArrayList, HashMap, TreeMap, HashSet, queues and sorting with Comparator on real tasks.7 problems
• Java Object-Oriented Programming PracticePractise Java classes, constructors, encapsulation, inheritance, polymorphism, interfaces, records and enums.6 problems

Advanced Advanced sets

• Advanced Java PracticePractise Java exceptions, file handling, generics, lambdas, streams, Optional and concurrency.8 problems

Beginner Beginner sets

• HTML Basics PracticePractise HTML document structure, headings, paragraphs, text formatting, links, images and lists. Each solution is rendered live so you can compare it with yours.7 problems

Intermediate Intermediate sets

• HTML Forms and Tables PracticePractise building data tables and accessible forms with labels, the right input types and built-in validation. Each solution is rendered live, and the forms show what they would send.6 problems
• Semantic HTML and Accessibility PracticePractise semantic page structure, landmarks, accessible images, links and buttons, and built-in interactive elements. Each solution is rendered live.6 problems

Advanced Advanced sets

Advanced HTML sets are being written.

Beginner Beginner sets

• CSS Selectors and Box Model PracticePractise CSS selectors, specificity, colours, units and the box model by styling given HTML. Each solution is rendered live next to the HTML it styles.6 problems

Intermediate Intermediate sets

• CSS Layout Practice: Flexbox, Grid and PositionPractise building real layouts with Flexbox, CSS Grid and positioning, from navigation bars to card grids and badges. Each solution is rendered live.6 problems
• Responsive CSS and Effects PracticePractise media queries, fluid sizes, custom properties, transitions and animations by styling given HTML. Each solution is rendered live; resize your browser window to test the responsive ones.6 problems

Advanced Advanced sets

Advanced CSS sets are being written.

Beginner Beginner sets

• JavaScript Basics PracticePractise JavaScript variables, numbers, strings, conditions and loops with small programs. Every solution has been run with Node.js and shows its real output.7 problems

Intermediate Intermediate sets

• JavaScript Arrays and Objects PracticePractise transforming data with map, filter, reduce and sort, working with objects, destructuring, Map, Set and JSON. Every solution has been run with Node.js and shows its real output.7 problems
• JavaScript Functions PracticePractise writing functions, default and rest parameters, arrow functions, callbacks and closures. Every solution has been run with Node.js and shows its real output.6 problems

Advanced Advanced sets

• Asynchronous JavaScript PracticePractise timers, promises, async/await, error handling and running work in parallel. Every solution has been run with Node.js and shows its real output.6 problems

Beginner Beginner sets

• TypeScript Basics PracticePractise type annotations, arrays and tuples, typed functions, object types and optional properties. Every solution has been type-checked in strict mode and run, and shows its real output.6 problems

Intermediate Intermediate sets

• TypeScript Unions, Narrowing and Interfaces PracticePractise union and literal types, narrowing, discriminated unions, interfaces, classes and type guards. Every solution has been type-checked in strict mode and run, and shows its real output.6 problems

Advanced Advanced sets

• TypeScript Generics and Utility Types PracticePractise generic functions and classes, constraints, keyof, utility types and mapped types. Every solution has been type-checked in strict mode and run, and shows its real output.6 problems

Beginner Beginner sets

• DSA Arrays, Strings and Hashing PracticePractise hash maps, two pointers, sliding windows and prefix sums on classic array and string problems.8 problems

Intermediate Intermediate sets

• DSA Linked Lists, Stacks, Trees and Heaps PracticePractise building and manipulating linked lists, stacks, binary trees, binary search trees and heaps.8 problems

Advanced Advanced sets

• DSA Graphs, Backtracking and Dynamic Programming PracticePractise BFS, DFS, shortest paths, topological sort, backtracking and dynamic programming.8 problems

Beginner Beginner sets

• NumPy and pandas Basics PracticePractise NumPy arrays, creating and inspecting DataFrames, selecting, filtering, sorting and adding columns.7 problems

Intermediate Intermediate sets

• Data Cleaning and Analysis PracticePractise cleaning missing values, types, text and duplicates, then grouping, pivot tables and merging with pandas.8 problems

Advanced Advanced sets

• Statistics and Time Series PracticePractise descriptive statistics, outliers, correlation, hypothesis tests, A/B tests, dates and rolling windows.8 problems

Beginner Beginner sets

• React Basics PracticePractise JSX, components, props, conditional rendering, lists with keys, events and useState. Each solution runs live so you can compare it with yours.6 problems

Intermediate Intermediate sets

• React Forms and Hooks PracticePractise controlled forms, updating arrays in state, lifting state, useEffect with cleanup, useRef, useReducer, context and custom hooks. Each solution runs live.6 problems

Advanced Advanced sets

Advanced React sets are being written.

Beginner Beginner sets

• Node.js Core PracticePractise Node.js core modules with real scripts. Files, JSON, paths, URLs, command-line arguments, events, streams and asynchronous code. Every solution has been run with Node.js and shows its real output.7 problems

Intermediate Intermediate sets

• Node.js Back-End PracticePractise back-end building blocks with runnable Node.js scripts. Validation with Zod, SQL with node:sqlite, transactions, password hashing, JWTs and an HTTP server tested with fetch. Every solution has been run and shows its real output.7 problems

Advanced Advanced sets

Advanced Node.js and Express sets are being written.

Beginner Beginner sets

• Regression and Classification PracticePractise train/test splits, linear and logistic regression, regression and classification metrics, k-nearest neighbours and decision trees with scikit-learn.7 problems

Intermediate Intermediate sets

• Building Better Models PracticePractise ensembles, feature engineering, pipelines with mixed columns, cross-validation, hyperparameter tuning and imbalanced data with scikit-learn.7 problems
• Clustering, PCA, Text and Production PracticePractise k-means, PCA, anomaly detection, TF-IDF text classification, cosine similarity, saving models and drift checks.7 problems

Advanced Advanced sets

Advanced Machine Learning sets are being written.

Beginner Beginner sets

• Git Basics PracticePractise everyday Git: commits, the staging area, history, diffs, .gitignore, undoing changes, branches, merging and conflicts. Every solution has been run with Git and shows its real output.12 problems

Intermediate Intermediate sets

• Git Team and Recovery PracticePractise team Git with a local remote: push and pull, rejected pushes, rebasing, interactive clean-up, cherry-pick, tags, the reflog, bisect and hooks. Every solution has been run with Git and shows its real output.11 problems

Advanced Advanced sets

Advanced Git and GitHub sets are being written.

Beginner Beginner sets

• Linux Commands PracticePractise everyday Linux commands: files, searching, pipes, text tools, permissions and processes. Every solution has been run on Ubuntu and shows its real output.14 problems

Intermediate Intermediate sets

• Bash Scripting PracticePractise Bash scripting: variables, conditions, loops, functions, arguments, arrays and safe scripts. Every solution has been run on Ubuntu and shows its real output.12 problems

Advanced Advanced sets

Advanced Linux and Bash sets are being written.

Beginner Beginner sets

Beginner Docker and Kubernetes sets are being written.

Intermediate Intermediate sets

• Docker PracticePractise Docker: running containers, writing Dockerfiles, volumes, networks and Compose. Every solution has been run with Docker on Ubuntu and shows its real output.12 problems

Advanced Advanced sets

Advanced Docker and Kubernetes sets are being written.

Beginner Beginner sets

Beginner System Design sets are being written.

Intermediate Intermediate sets

• System Design PracticePractise the calculations and building blocks behind system design: estimates, availability, caches, rate limiters, hashing, quorums and percentiles. Every solution has been run with Python and shows its real output.12 problems

Advanced Advanced sets

Advanced System Design sets are being written.